A malaria parasite exclusive to whitetails has been identified in up to 24 percent of the eastern U.S. deer population, a new scientific study concludes.

Ellen Martinsen, an ornithologist and post-doctoral researcher working at the Smithsonian Conservation Biology Institute’s genetics center made the discovery of the malaria parasite in deer accidentally, while researching mosquitoes that have infected birds at the National Zoo in Washington, D.C., with various strains of malaria.

“We stumbled across the malaria (in deer),” she told the Northern Virginia Daily. “We weren’t looking for it.”

She had identified 21 species of mosquitoes living in the zoo, but only one, Anopheles punctipennis, carried the type of malaria discovered in the deer.

“What we were doing was catching and screening the mosquitoes from the Smithsonian’s National Zoo as part of a bird project,” Martinsen told Mainenewsonline.com. The project began two years ago and the stunning results were published last week in the journal Science Advances in February.

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Robert Fleischer, head of the Center for Conservation and Evolutionary Genetics and co-author of the study, said “no deer in the study was showing symptoms of illness.”

Over 100 species of malaria micro-organisms have been discovered in the genus Plasmodium. Malaria parasites can infect birds, reptiles and some mammals, but only four species infect humans, primarily falciparum, which kills about 1 million people worldwide each year, according to the CDC.

The variety of malaria parasite found to be widespread in deer in the new study, Plasmodium odocoilei, had been identified previously only one time, in one single whitetail in Texas in 1967.


Two related species of malaria were actually discovered in the deer. It’s believed they might have diverged genetically over 1 million years ago. The identified strain, also found in the Texas deer 49 years ago, is believed to date back to when ancestors of the current whitetail crossed the Bering Land Bridge around 6 million years ago.

Ultimately, the malaria variety likely dates back to one found in Old World bats, which it appears closely related to.

Martinsen said the parasite levels in the blood are so low that they are undetectable by traditional techniques with a light microscope. The parasites were discovered at a level of about one per 65,000 red blood cells by the use of polymerase chain reaction technology, a method that amplifies DNA to make it easier to decipher.

A total of 308 deer were sampled from 25 counties in 17 states. Deer in 10 states had the malaria, ranging from New York to Louisiana, in 17 different counties. Over 40 deer, or 18 percent, carried the parasite. In Virginia and West Virginia the level was higher at 25 percent.

“Technically,” Smithsonian Science noted, “this is a rediscovery of the same Plasmodium odocoilei parasite that was found in a whitetail deer before — but only once, in a single specimen in Texas that had its spleen removed in 1967 to allow hidden pathogens to manifest.”

Using PCR technology, Martinsen compared blood smears from current samples with those from a slide made of blood from the infected 1967 deer, on loan from the Natural History Museum in London, to positively identify the malaria parasite as P. Odocoilei.

“It hasn’t been detected in the interim because people were looking with a microscope and the parasite resides in the blood at such low concentrations that it is virtually invisible,” Martinsen said.

The parasite was discovered in two individual mosquitoes that happened to have a lot of blood. They then scanned it for vertebrate genes and traced it back to its origin in whitetail deer. Blood samples from repositories were used to check other animals including elk, pronghorn, and mule deer.

The parasite was not discovered in those animals or whitetails in the West.

While the researchers also detected a genetic marker for a second species of malaria in the deer studied, so far it hasn’t been identified.

Smithsonian Science notes that “low-level infections of malaria in vertebrates elsewhere in the world are correlated with reduced fertility and shorter lifespans.” These malaria strains have been present in deer for many years, however, so they are not likely related to the declines seen in whitetail populations in some areas in recent years.

Co-author Fleischer says the risk of transmission of this form of malaria to humans is extremely low. Only four of the 100-plus other species of malaria discovered by scientists affect humans.

“If it hasn’t yet switched hosts and jumped to humans, it is unlikely to do so now,” he said.

The discovery has shocked the science world, since whitetails are one of the most intensively-studied wild animals in the world.